What is a good way to turn straight edges to curves?
USING PROPORTIONAL EDITING


Add some edge loops (Ctrl+R) in the middle of the object to devide the object evenly. Select the face loop in the middle (Alt+RMB) and snap the cursor to it (Shift+S--&gt;Cursor to Selected). Enable proportional editing, then press S,Shift+Z to scale along X and Y axes only.


USING SIMPLE DEFORM MODIFIER (STRETCH)


Give the object a Simple Deform modifier, change its mode to Stretch and adjust the deform factor as you like.


USING LATTICE MODIFIER


Add a Lattice, scale it to fit the object's size and subdivide it a few times. Select the object and give it a Lattice Modifier. Select the points around the middle subdivision of the lattice, then scale them with S.
NOTE: Adding more subdivision to the lattice allows you to better control the shape of the object (you can make it more uneven/realistic).